The present invention relates to a system, method, and computer program product for controlling maneuverable wheels on a vehicle, and, more particularly, to a method, system, and computer program product for a controlling a plurality of steerable wheels on a vehicle in order to individually steer each maneuverable wheel according to a steering radius and a crab angle.
Vehicles require a high degree of ground maneuverability. Large vehicles tend to have limited maneuverability due to many factors. Many of the present steering system designs for such vehicles, however, do not always accommodate some of desired maneuvering situations. For example, aircraft typically taxi to a position at an airport where portable docking equipment is attached for loading and unloading. Some examples of portable docking equipment include passenger jetways, cargo conveyors, lifts, loaders, etc. To date, docking equipment must be portable due the inability of aircraft to precisely maneuver about fixed equipment or structures. The capabilities of docking equipment are limited due to size and weight constraints necessary to maintain portability. Thus, fixed docking equipment that accommodates aircraft has not been designed. If aircraft could precisely maneuver with respect to fixed structures then the potential for more complex docking equipment may be realized. Eliminating the size, weight, and portability requirements permits more complex systems, perhaps linking multiple airplanes, trucks, trains, and so forth. Such a system has the potential for high throughput and extensive automation, both factors in high efficiency. Therefore, for these and other reasons it would be advantageous to improve the ground maneuverability of many vehicles, including aircraft.
Ground maneuverability of many large vehicles is accomplished by a single wheel or set of wheels along the forward portion of the vehicle. Large transport vehicles, in particular, require a plurality of wheels, fore and aft, to effectively support a payload. These vehicles typically have very long wheelbases. Turning radius is increased with wheelbase on vehicles that steer by pairs of wheels, with pairs or multiple pairs of wheels tracking behind, cars and trucks for example. A large turning radius compromises maneuverability of such vehicles in close quarters, thus restricting their capacity to operate in confined spaces.
Large air transports, are also limited in maneuverability, however, this has more to do with typical wheel arrangements. Low wing transports that tend to align the landing gear along a spanwise axis arrange the tires in groups that are displaced fore and aft from the average spanwise axis of the landing gear. A typical grouping is two by two. These gears do not steer. Steering is provided by forward-mounted nosewheels. The ground maneuverability of aircraft with such landing gear is compromised by tire scrubbing on the non-maneuverable landing gears. The assembly of the wheels, tires, and interconnecting structure on landing gear is usually called a xe2x80x9cbogeyxe2x80x9d or xe2x80x9ctruckxe2x80x9d. The fore/aft displacement of the parallel wheels produces tire scrubbing. With such bogeys, there is no single intersection of the nosewheel and main gear axes.
A degree of scrubbing results from a fore/aft gear separation and is dependent primarily on the fore-aft dimension. An additional scrubbing factor is the track between the extreme left and right gears. A wide track gear will place the inboard gear close to the turn center point, exacerbating the scrubbing on the inboard gear. There are a few airplanes with six wheel bogiesxe2x80x94three pairs of wheels. The large wheelbase of such a bogey reduces maneuverability to unacceptable levels if all wheels are fixed. The typical solution is to steer the rear pair of wheels on the bogey driven by the same steering input provided for the nose gear.
Nose wheel to main gear wheel base, that is the longitudinal distance between the nose gear axle and the main gear axles, also affects maneuverability, albeit to a smaller extent. When the airplane is steered by the nose gear alone, the main gear do not follow the same track as the nose gear. Instead, the main gear tends to track inside the path of the nose gear. This increases the effective width of the gear if the nose gear follows the taxiway centerline. To date, the common solution to this problem is to use only airports that have sufficiently large runways, taxiways, and aprons to allow for large steering maneuvers.
Some very large high-wing transports use main landing gears that are arrayed in a longitudinal direction along the fuselage sides at an average position that is behind the center of gravity. Steering is also provided by a forward-mounted nose wheel. Due to the longitudinal extent of the main gear, large turning diameters and increased tire scrubbing seriously reduce ground maneuverability. Some aircraft solve this problem by allowing the aft main gear struts to castor.
Other high-wing transports, such as the B-52 bomber, have main landing gear that is located well in front of the center of gravity and additional landing gear located well behind the center of gravity to make room for a disposable payload in a bomb bay. The B-52 has four main gear struts, two forward and two aft of the bomb bay. On each strut are two coaxial wheels. The angle of each pair can be controlled. For steering, only the forward pairs are controlledxe2x80x94the aft pair remains fixed. For crosswind landings and takeoffs the angle of all struts are moved in concert so that the axles are parallel. The principal shortcoming is that it is very narrow so that the airplane tends to tip over. The B-52 uses additional small, castering outrigger landing gears near the wing tips to prevent tip over. This limits the B-52 to very wide runways.
There are some ground transport vehicles with a plurality of wheels that can be steered. Most of these are steered only with the forward pair of wheels. This results in substantial tire scrubbing and limits the minimum turning radius. Some large trucks solve the maneuverability problem by introducing articulation between the tractor and the trailer, but articulation is not practical in many vehicles.
One successful attempt to improve steering on large ground transports has been effective on the Tunner 60K Loader, built by Systems and Electronics, Inc., of St. Louis, Mo. The 60K Loader is a 20-wheel cargo handling vehicle with ten pairs of wheels arrayed in two longitudinal columns and five rows. The forward two rows and aft two rows steer. The driver, via a mechanical linkage, provides steering command. If the design were applied to other vehicles, such as aircraft, the 60K loader design has limited maneuverability chiefly because the suspension geometry and proximity of the chassis limit the angle to which the wheels can be turned. The mechanical linkage additionally limits the maximum wheel turning angle. When steering the fore and aft gear the vehicle follows a common track, and the rear wheels do not track inside the front wheels. The 60K loader can pivot only about a point that is collinear with the fixed middle wheels. As such, a chief limitation of the 60K loader design is the inflexible steering geometry. Additionally, the 60K loader does not have any ability to steer to a crab angle. Therefore, when applied to other vehicles, the 60K loader design is limited by the heavy steering linkage, the inability to crab, and the inability to pivot about any point not on the axis of the fixed wheels.
Aircraft face an additional maneuverability problem with respect to crosswinds on takeoffs and landings. Almost all large air transports use one of two techniques for landing in a crosswind. In the first technique the airplane is flown wings-level with the airplane crabbed with respect to the ground track to account for the crosswind component. At the last moment before touchdown, the pilot uses the rudder to yaw the airplane and landing gear so that it is better aligned with the runway axis. This is a challenging maneuver to time and perform, and requires skill on the part of the aircraft pilot. If the airplane touches down at an angle to the runway axis, a rapid correction must be made to avoid running off the runway.
In the second technique, the airplane is flown in a sideslip condition so that the airplane is at all times aligned with the runway axis. To compensate for the sideforce generated by the yaw angle, the pilot must bank the airplane to the opposite direction from the yaw. This technique requires no last-second maneuver to align the aircraft with the ground track and the runway axis, however, the airplane touches down with one wing exceptionally low to the ground and often touches down on just a single main landing gear. This maneuver is the one used by most transport aircraft pilots, however, it is limited by low wing dihedral angles. Aircraft designers have compensated for this technique by designing aircraft with considerably higher wing dihedral angles than might otherwise be used. In particular, this has especially limited the development of ground effect aircraft that strongly favor aircraft with low dihedral angles.
Wing dihedral angle for many transport aircraft is also set to provide wing tip clearance for crosswind takeoffs. Low wing dihedral angles are proscribed due to the possibility of striking a wing tip just as the airplane leaves the ground. This is the result of several factors. In a crosswind takeoff, the airplane tends to roll away from the wind due to a yaw-roll couple influenced by wing dihedral, the fuselage and the vertical tail. This tendency to roll is countered by a preset opposite aileron roll input by the pilot. The exact canceling input is difficult for the pilot to judge before takeoff because of the stabilizing influence of the gear track on the roll attitude. As the airplane rotates and takes off, this stabilizing influence is suddenly eliminated so that the airplane is free to roll. If the pilot has not properly set the aileron deflection, the plane will tend to roll until a correction is made. This correction is complicated by the yaw stability of the airplane. When the airplane is on the takeoff roll in a crosswind, the airplane is yawed with respect to the airstream. As soon as it takes off, the airplane, due to its yaw stability, tends to align itself with flow. This yawing motion quickly changes the influence of the wing dihedral on roll so that the compensating aileron input changes rapidly in the first second or two after takeoff. To summarize, it is difficult for the pilot to judge the correct aileron compensation before takeoff and to respond rapidly with adjustments just after takeoff. An error in compensation or response can result in a wing tip strike. As a result of this problem, aircraft wing dihedral angles are designed to provide the pilot a statistically safe margin.
As presented above, ground maneuverability and crosswind takeoff and landing maneuverability are primary obstacles to the design of larger air transport. Therefore there is a need in the art to enable an extremely large and heavy aircraft to operate from airports designed for smaller, lighter aircraft. There is also a need in the art to permit a reduction in wing dihedral angle in order to enhance the efficiency of wing-in-ground-effect flight. Additionally, with respect to ground vehicles and aircraft, there is a need in the art to provide a high level of ground maneuverability including the ability to turn about a small radius and to follow a curved path with a narrow effective track.
An improved method, system, and computer program product are therefore provided for controlling a plurality of maneuverable wheels on a vehicle in order to increase ground maneuverability and to compensate for crosswind takeoffs and landings. In this regard, the method, system, and computer program product of one aspect of the present invention increase ground maneuverability by providing a steering control interface and a crab control interface. The steering control interface elects a steering radius for the vehicle. The steering radius is measured from a reference point relative to the vehicle to a point on the ground plane. The crab angle of the vehicle is the angular difference between the true heading of the aircraft and the straight ground track of the aircraft. The true heading is the orientation of the longitudinal axis of the aircraft with respect to geometric heading. The steering control interface and crab control interface are generally rotating hand wheels accessible to the pilot of the vehicle. A processor receives the input from both the steering control interface and the crab control interface and determines a steering angle for each of the plurality of maneuverable wheels.
Each wheel is controlled by a steering mechanism responsive to the steering angle and mechanically turns each respective wheel according to its steering angle. The steering angle for each wheel is chosen in order to rotate each wheel concentrically about the same point in order to minimize tire scrubbing. In one embodiment, a computer program product may control the processor in order to determine the steering radius and the crab angle of the vehicle and to calculate a steering angle for each of the plurality of wheels. The steering angle may be calculated based on the position of each wheel relative to the reference point of the vehicle, the steering radius and the crab angle.
An additional embodiment of the method and system for controlling a plurality of maneuverable wheels on a vehicle includes a braking control interface. As such, the braking control interface-also provides input to the processor such that the processor can determine steering angles in order to artificially increase tire scrubbing to slow the vehicle. In this embodiment, the steering angles of each of the plurality of wheels can be determined to increase the efficiency of other braking mechanisms on the vehicle.
Another aspect of the method, system, and computer program product for controlling a plurality of maneuverable wheels provides compensation for crosswinds on aircraft landing. As such, it is desirable to maneuver the aircraft""s true heading towards a relative wind, which is not necessarily aligned with the axis of the runway. While the aircraft is in flight, the difference between the true heading and the ground track of the aircraft is called the drift angle. The crosswind causes the difference in the ground track of the aircraft. This drift angle requires compensation corresponding to a necessary crab angle of the aircraft that will be required as the aircraft touches down on the runway. Thus, a crab angle may be computed by determining the difference between the true heading of the aircraft and a ground track of the aircraft, and each of the plurality of maneuverable wheels may be steered to a particular steering angle corresponding to that crab angle. Therefore, the aircraft lands with no sideslip angle while maintaining a ground track corresponding to the runway axis.
Similarly, another aspect of the method, system, and computer program product controls a plurality of maneuverable wheels on an aircraft to compensate for crosswind takeoffs by determining a crab angle. The desired crab angle of the aircraft may be determined by onboard instrumentation, which provides the aircraft airspeed, wind speed and wind direction. The calculated crab angle may be chosen in order to align the true heading of the aircraft into the direction of relative wind while maintaining the ground track of the aircraft corresponding to the runway axis. However, to obtain maximum acceleration of the aircraft prior to achieving takeoff, the crab angle may be maintained at zero and the aircraft only maneuvered to a crab angle corresponding to the relative wind direction just prior to takeoff. Therefore, at a predetermined airspeed just prior to takeoff, each of the plurality of landing gear are used to orient the aircraft to the desired crab angle while maintaining a constant ground track down the runway.
In one advantageous embodiment of the method and system for controlling a plurality of wheels on an aircraft, the wind angle and relative wind speed may be provided by instrumentation commonly found on aircraft such as pitot-static tube airspeed sensors and wind direction sensors. In another aspect of the method and system, the crab angle may be calculated based upon expected remotely sensed wind speed and remotely sensed wind direction determined by sensors located remotely from the aircraft. Such sensors are commonly found at airports and such data is typically provided to the aircraft by radio communications.
Additionally, as aircraft airspeed increases with acceleration toward takeoff, relative wind speed and wind direction correspondingly changes. Therefore, one aspect of the method and system for controlling a plurality of wheels on an aircraft, calculates an estimated aircraft airspeed at takeoff to determine the necessary crab angle at takeoff. As such, the crab angle and steering angle for each of the plurality of maneuverable wheels may be determined in advance. Additionally, this provides for time integration of relative wind direction such that the calculated crab angle may be refined prior to takeoff.
Therefore one aspect of the method, system, and computer program product enables an extremely large and heavy aircraft to operate from airports designed for smaller, lighter aircraft. Also, increased maneuverability may permit precise aircraft maneuvering about fixed structures, and thus allow design of more efficient docking systems. It is also advantageous to achieve increased maneuverability by using approximately ordinary landing gear commercially available to aircraft manufacturers, without resorting to costly new landing gear systems or supplemental maneuvering equipment. Another aspect of the method, system, and computer program product permits a reduction in wing dihedral angle in order to enhance the efficiency of wing-in-ground-effect flight. In one sense, this is achieved by crabbing the landing gear for takeoff and landing so that the need or tendency to roll is greatly reduced. Another aspect of the invention provides a high level of ground maneuverability including the ability to turn about a small radius and to follow a curved path with a narrow effective track. This aspect is generally achieved by effectively controlling the steering angle of a plurality of wheels in order to precisely turn the vehicle about a given point.